A double-row tapered-roller bearing assembly has been used as a bearing for an automotive wheel and the like. FIG. 10 is a sectional view of the double-row tapered-roller bearing assembly conventionally used widely in the art (Conventional Example 1). The conventional example 1 includes: a single outer ring 1 having two outer raceways 2, 2; a pair of inner rings 4, 4 having inner raceways 3, 3; and plural tapered rollers 5, 5 rollably interposed between the outer raceways and the inner raceways. The conically recessed outer raceways 2, 2 are formed on an inner peripheral surface of the outer ring 1. These outer raceways 2, 2 are disposed in opposed relation so that an inside diameter of the outer ring 1 is progressively decreased from either end thereof toward the center thereof. The inner rings 4, 4 are individually formed with the conically protruded inner raceways 3, 3 in correspondence to the respective outer raceways 2, 2. The tapered rollers 5, 5 are clamped between the inner rings 4, 4 and the outer ring 1 as interposed between the inner raceways 3, 3 and the outer raceways 2, 2 and are arranged as regularly spaced by means of a cage 6. Seal rings 7, 7 are disposed between the opposite ends of the inner peripheral surface of the outer ring 1 and a respective end of outer peripheral surfaces of the inner rings 4, 4. The seal rings 7, 7 are disposed for the purposes of hermetically sealing space between the outer ring 1 and the inner rings 4, 4 thereby preventing the invasion of external foreign substances (dusts, water, metal powder and the like) and the leakage of lubricant.
The aforementioned double-row tapered-roller bearing assembly of the conventional example 1 is assembled as follows. First, as shown in FIG. 11, sub-assemble members 11 each combining the cage 6 with the tapered rollers 5 are assembled to the inner ring 4. The resultant inner ring is inserted into the inner periphery of the outer ring 1 so as to mate the tapered rollers 5 with the outer raceways 2, 2 and the inner raceways 3.3, as shown in FIG. 12. Finally, the seal rings 7, 7 are fitted in the respective spaces between the opposite ends of the inner peripheral surface of the outer ring 1 and the respective ends of the outer peripheral surfaces of the inner rings 4, 4.
As shown in FIG. 13, the cage 6 substantially defines a truncated conical surface including: a greater ring 6a and a smaller ring 6b as a pair of ring members in spaced relation; a plurality of ridges 6c bridged between these ring members; and an annular rib 6d extended diametrically inwardly from an diametrical inside end of the smaller ring 6b. In such a construction, a trapezoidal window pocket 10 for accommodating the tapered roller 5 is defined between a respective pair of adjoining ridges 6c such that the plural trapezoidal pockets are arranged with equal spacing. FIG. 14 is a sectional view showing the sub-assemble member 11 with the tapered rollers 5 accommodated in the pockets 10. The tapered rollers 5 are disposed in a manner to sink into the respective pockets 10 from a diametrical inner side of the cage 6. FIG. 15 is a sectional view of the ridges 6c of the cage 6 taken on the plane perpendicular to a longitudinal direction of the ridges. The tapered rollers 5, disposed as described above, are depicted with the phantom lines. A circumferential width W10 of the pocket or the space between each pair of adjoining ridges 6c is smaller than a diameter of the tapered roller 5 as determined at each place on a generatrix of the tapered rollers 5. Hence, the tapered rollers 5 are held in posture as sunk into the respective pockets of the cage 6. In the sub-assemble member 11, the cage 6 merely supports the tapered rollers 5 on diametrical outer sides thereof. Hence, the tapered rollers 5 never drop from the sub-assemble member 11 diametrically outwardly thereof. Depending upon some posture of the sub-assemble member 11, however, the tapered roller 5 may incline diametrically inwardly to drop off from the sub-assemble member 11. A cage having the structure detrimentally allowing the disposed tapered rollers to drop diametrically inwardly, as exemplified by this cage 6, has high general-purpose versatility but also has a drawback that the tapered rollers are apt to drop off.
Since the conventional example 1 adopts the aforementioned assembling method as shown in FIG. 11 and FIG. 12, the drop-off of the tapered rollers 5 is not encountered during the assembling steps. This is because the sub-assemble member 11 can be handled in a state where the sub-assemble member 11 is assembled with the inner ring 4, as shown in FIG. 11. That is, the tapered rollers 5 in the state of FIG. 11 are clamped between the inner ring 4 and the cage 6 so that the tapered rollers are retained as prevented from dropping off.
More recently, a hub unit having a so-called third generation structure has been proposed aiming at reducing costs by reducing the number of components. FIG. 16 illustrates an example of a conventionally known structure of the hub unit (Conventional Example 2). Unlike the conventional example 1, this hub unit is constructed such that one end of a hub body 16 is formed with a flange 26 for supporting a road wheel, that an outer peripheral surface of an intermediate portion of the hub body 16 is formed with a first inner raceway 28 constituting a first-row tapered-roller bearing, and that the other end of the hub body 16 is adapted to allow an inner ring 35 having a second inner raceway 36 to be fitted on an outer periphery thereof. Such a structure cannot adopt the assembling step as suggested by the conventional example 1, wherein a sub-assemble member 19, combined with the hub body 16 defining the inner ring, is inserted into an outer ring 13. In the case of the conventional example 2 shown in FIG. 16, the flange 26 formed at the one end of the hub body 16 interferes, thus disabling the mounting of a first seal ring 15 to place near the flange after insertion of the hub body 16 into the outer ring 13. Hence, the following steps are taken in this case. First, the sub-assemble member 19 having first-row tapered rollers 18a disposed in a first cage 17 is placed on a first outer raceway 22 of the outer ring 13. Next, the first seal ring 15 is mounted to place. Subsequently, the hub body 16 is inserted into the outer ring 13. In the case of the structure of the conventional example 2, it is impossible to handle the sub-assemble member 19 in a state where the sub-assemble member is assembled with the hub body 16 defining the inner ring.
When the hub body 16 is inserted into the outer ring 13 according to the conventional example 2, either one of the following methods is taken. (1) The outer ring 13 with the sub-assemble member 19 and the first seal ring 15 mounted thereto is postureed to direct a rotary axis of the bearing horizontally and then, the outer ring 13 is combined with the hub body. (2) The outer ring 13 with the sub-assemble member 19 and the first seal ring 15 mounted thereto is postureed such that the rotary axis of the bearing is directed vertically while a second outer raceway 23 is postureed above the first outer raceway 22 and then, the outer ring 13 is combined with the hub body 16. The reason for taking the method (1) or (2) is because it is more efficient to move the lighter component when combining these components. Another reason for taking the method (2) is as follows. In the subsequent step, the inner ring 35 is press-fitted on the hub body 16. In order to receive an axial force associated with the press-fit, the hub body 16 is placed on a jig as upstanding on its side formed with the flange 26. According to the method (2), the hub body may be subjected to this step in an as-is state, so that the operation efficiency is increased. Unfortunately, the methods (1) and (2) cannot obviate the drop-off of the first-row tapered rollers 18a. 
In order to overcome this problem, the conventional example 2 devices the structure of the first cage 17 which is designed as an anti-drop cage. In a state where the first-row tapered rollers 18a and the first cage 17 are combined into the sub-assemble member 19, the tapered rollers 18a are retained by the first cage 17 so as to be prevented from dropping diametrically inwardly. FIG. 9 is a sectional view showing ridges 17c of the first cage 17 of the conventional example 2. In the figure, the tapered rollers 18a so retained are depicted with the phantom lines. Referring to FIG. 9, the width W17 of a pocket in section, as viewed from the diametrical inner side of the first cage 17, is smaller than a diameter D18a of the tapered roller 18a in section. Thus, the tapered roller 18a is so retained as not to drop diametrically inwardly (see, for example, Japanese Unexamined Patent Publication No. 2000-65049).